The present invention relates generally to protection of transistors connected in a switching bridge configuration and more specifically to protection for the series connected transistors therein.
Transistors connected in series between terminals of a power supply are often found in switching bridge control systems for brushless DC motors. Typically, brushless motors are constructed with stationary windings and a rotating permanent magnetic field. The windings are commutated by the operating transistor pairs which are controlled in response to signals from rotor position sensors. Since the brushes and commutator of the normal DC motor are eliminated, and since commutation is controlled by the series connected transistors, brushless motors require sophisticated electronic motor control systems. A representative example of a DC motor having an electronic control system is disclosed in U.S. Pat. No. 4,368,411, entitled "Control System For Electric Motor", issued Jan. 11, 1983, to H. Keith Kidd.
In brushless motor circuits there are typically six transistors connected in a bridge configuration of three pairs of series connected transistors which are connected between terminals of a power supply. The series connected transistors should never be turned on together, since simultaneous operation causes a short circuit across the power supply. Bipolar transistors have a time delay between the time that they are commanded to turn off and the time that they actually do turn off. This time delay is caused by the charge stored in the base-emitter junction of the transistor. The time delay is commonly referred to as the storage time of the transistor. The turn on time of the transistor is generally much shorter than the turn-off time because of the storage time. If a series connected pair of transistors is switched rapidly from the state where one transistor is on to the state where the other transistor is on, the difference between storage time of the transistor being turned off and the turn-on time of the transistor being turned on will cause both transistors to be on for the period of the time difference, resulting in excessive currents which can damage the transistors.
An object of the present invention is to provide a protection circuit for such series connected transistor pairs.
Another object of the present invention is to provide a motor control circuit in which time delays are introduced where needed to prevent excessive currents and consequent damage to the series connected transistor pairs.